The present invention relates to colored powder, the individual particles of which contain a thermoplastic resin, possibly a magnetically attractable material and fluorescent dye. Powders of this kind are used, inter alia, in electrographic, electrophotographic and magnetographic image-forming processes. These types of powders, hereinafter also referred to as toner powders, are described inter alia in European Patent Application No. 0 350 099. In addition to possible other additives, toner powders according to this European Patent Application contain thermoplastic resins, finely distributed magnetically attractable pigments, such as carbonyl iron, and at least one yellow-fluorescent dye which has a strong fluorescence in the thermoplastic resin. The highly fluorescent yellow dyes are required to provide a color of acceptable brightness and color saturation to the toner powder, despite the very dark, magnetically attractable pigment present therein. In combination with the highly fluorescent yellow dye, other pigments or (fluorescent) dyes may be present to give the toner powder the intended color.
Examples of fluorescent dyes which may be present in the toner powder according to EP 0 350 099 are Rhodamine B (C.I. No. 45170), Basonyl Rot 560 (C.I., Basic Violet 11:1), Astra Phloxine (C.I. No. 48 070), Macrolex Fluorescent Yellow B10GN (C.I. Solvent Yellow 160:1), Thermoplast f-Gelb (C.I. No. 59 075) and Maxilon Brilliant Flavine 10 GFF (C.I. Basic Yellow 40).
In the preparation of toner powders according to the above European Patent Application, wherein magnetic pigment and fluorescent dye or dyes are finely distributed in the resin melt, a phenomenon which repeatedly occurs is that the fluorescence of the fluorescent dyes falls off sharply and finally colored toner powder of inadequate color quality is obtained.
This disadvantage occurs particularly in the preparation of red or magenta toner powder using the dyes of the type C.I. Basic Violet 11:1, such as Basonyl Rot 560, and fluorescence extinction is observed now and then during toner preparation in the case of other fluorescent dyes, such as the yellow-fluorescent dyes referred to in EP 0 350 099.
It has not yet been possible to ascertain the exact incidence of the fluorescence extinction, but it is clear that the effect is produced by the presence of iron ions, for example from the magnetic pigment, and that it is also influenced by the temperature level of the resin melt.